This invention relates to monitoring weld voltages. In particular, it relates to monitoring the voltages produced in AC resistance welding of the longitudinal seams of tubular 3-piece cans.
The longitudinal seams of 3-piece cans are often welded in machines in which the piece of metal forming the can body is folded such that its ends overlap and pushed along a member known as a z-bar or z-rail, into a welding position between two rollers. The z-bar serves, along its length, to bring the overlapping portions closer together and to control the degree of overlap. Copper or other welding wires are supported around each roller and are used as electrodes. An AC current is applied to the electrodes and welds the seams of the can body together in a series of spaced but overlapping weld nuggets, due to the AC voltage applied to the electrodes. Typical machines of this type are manufactured by SOUDRONIC AG, Switzerland, under their series designation FBB.
In order to determine the quality of welds it is desirable to monitor the voltage at the electrodes or the power absorbed by each weld nugget. Typical power monitors calculate an average power value for each nugget by monitoring the voltage and current in the welding circuit, multiplying these two wave forms and then, typically, integrating the results of a current cycle or more. A measure of the average power absorbed by each weld nugget is derived from this integration and this measure may be used to determine whether the weld has been successful or not. Clearly, if a weld has not been successful then the voltage and/or the power measured will be different. Such monitors are well known and typical ones include the CMB Engineering (Automation and Controls) models WM1, and M800. The voltage is generally monitored between one pick-up point adjacent the lower roller and a second pick-up point on the gimbal to which the upper weld roller is connected.
Two problems arise from this. Firstly, the voltage pick-up on the gimbal connected to the upper roller is remote from the actual electrode wire and hence does not truly measure the voltage which is passed through the overlapping seams on the can or other body. Errors can also arise by virtue of the rotating action of the roller. Secondly, the wire pick-up connected to the lower roller is passed along the length of the z-bar and emerges from its far end from whence a relatively long wire extends to the monitoring apparatus. Now, since the wire is relatively long and is in the vicinity of relatively high currents (typically around 4 kA rms) and is in effect a loop of wire present within this varying field, then electromagnetic induction occurs and the wire picks up noise and other interference.